Pests and Diseases of Greenhouse and indoor plants FAQs

Small cottony growths containing insects have appeared on my parlour palm (Neanthe bella). Can you identify these pests?

Your plants are infested with mealy bugs (Pseudococcus). These are sap-sucking insects which not only weaken the plants by their feeding but also secrete a sticky honeydew which later becomes covered with sooty moulds. Various species of this pest attack a wide range of greenhouse plants. Light infestations can be wiped off with a damp cloth, but heavy attacks are best dealt with by repeated sprays of malathion or a systemic insecticide.

Whitish powdery spots are developing on the young foliage of the ‘Black Hamburgh’ grapevine in my cold greenhouse. How can I prevent this from spreading?

Your vine is showing the early stages of infection by vine mildew (Uncinula necator). To prevent further spread of the disease, apply immediately a spray based on either benomyl or dinocap. Give repeat treatments but do not spray after thinning. Since the disease flourishes in stagnant air, good ventilation and adequate watering make disease-control easier.

When raising seedlings in my greenhouse I sometimes get poor emergence and suffer further loss of seedlings both before and after pricking out. How can I prevent these losses?

Both poor emergence and the later loss of seedlings can be attributed to attacks by soil-borne fungi. The first essential is to use clean seed trays and sterile John Innes Compost or one of the peat-based products. Dressing the seed with a proprietary fungicidal seed-dressing also helps. These measures, coupled with thin sowing and sensible watering, should prove successful. If damping-off occurs, the trays should be watered with Cheshunt compound.

Some of my potted primula plants seemed to be growing badly so I knocked one out of the pot and found that the roots were infested with insects like greenfly. What should I do?

These pests, which are rootaphids, can be troublesome on a wide range of greenhouse plants. Ideally, you should wash the roots free of soil and then dip them in a spray-strength general insecticide before repotting. An easier method is to give the pots a heavy watering with the insecticide solution.

The stems of my vine are encrusted with reddish-brown barnacle-like scales up to 6 mm (¼ in) long. Can you identify this trouble?

Your vines are infested with brown scale (Parthenolecanium comi), sap-sucking insects which remain fixed to the bark; like aphids, they produce sticky honeydew which becomes covered with sooty moulds. They can be scraped off the dormant rods in winter, but even so it may be necessary to apply summer sprays of malathion or pirimiphos-methyl. Various other species of scale insects attack a wide range of greenhouse plants.

Tomatoes planted in my greenhouse border showed poor growth and began to wilt, so I dug one up and washed the soil off the roots. The root system was poor, and I found numerous small insects floating on the water. Could these be the cause of the trouble?

Yes. Glasshouse centipedes (symphi;lids), which are white, wingless insects with 12 pairs of legs, can be very damaging. They feed on roots and may also damage leaves touching the soil. Apply either a soil insecticide before replanting or a heavy soil drench of spray-strength general insecticide.

Whitefly are attacking most of the plants in my greenhouse. I have tried spraying with a general insecticide but without success. What should I do?

Whitefly are difficult to control because their eggs and larval stages are resistant to most insecticides. The new pyrethroid insecticides permethrin and resmethrin, however, are effective against most stages. Even so, it is necessary to make three to four applications at 4-7 day intervals. Proprietary spray preparations of these insecticides are available; better still, use permethrin-based smoke cones.

One of my potted cinerarias grew badly and began to wilt, so I knocked it out of the pot and examined the roots. Amongst these I found a fat white grub with a dark head. What is this pest?

The grub is the larva of the vine weevil (Otiorhynchus sulcatus), a common pest of many plants, which not only feeds on the roots but may also tunnel into bulbs, corms, and tubers. You can guard against attack by working a soil insecticide into the compost; alternatively, apply a heavy soil drench of spray-strength insecticide based on HCH, malathion, or pirimiphos-methyl.

The leaves of various plants in my greenhouse have turned a sickly yellow colour and on some plants fine cobwebs are showing. What’s the trouble?

It looks as if your greenhouse is infested with red-spider mites (Tetranychus urticae); if you examine the lower-leaf surfaces carefully you will find that they are covered with tiny insects just visible to the naked eye. The mites spin silk threads to move around the plants. Control them by repeated sprays with systemic insecticides or with a greenhouse smoke based on pirimiphos-methyl.

Some of my house plants seem to have stopped growing and are tending to wilt in sunlight even when well watered. The only possible pests I can see are small dark-coloured gnats flying round the plants.

The small gnats are almost certainly fungus gnats (sciarid flies), which lay their eggs in the compost; the small maggots then feed on the plant roots, seriously affecting the plant growth. They can be controlled with a soil drench of a spray-strength insecticide such as HCH, malathion, or pirimiphos-methyl.

After planting out, my cucumber seedlings grew badly and began to wilt. On digging one up, I found large numbers of tiny grey insects jumping around in the soil. Could these have caused the damage?

From your description it seems that your cucumbers have been attacked by springtails (Collembola); some species of these soil-living wingless insects feed on plant roots, on stem bases, and also on any leaves touching the surface of the soil. Before replanting, work a soil insecticide into the top layer of compost, alternatively, apply a heavy soil drench of spray-strength general insecticide, such as those mentioned in 1013 above.